1. Field of the Invention
This invention relates to the field of machine control.
2. Description of Related Art
Proximity switches are well known devices used widely in machine control. The simplest proximity switch is a mechanical micro-switch which has a binary output. The switch is closed when a physical object mechanically moves the microswitch level. Today there are a number of sophisticated proximity detecting devices which may detect the presence or absence of an object without physical contact. In these devices, a field is emitted by a transducer and perturbations in the field caused by the presence of the object are sensed to determine the presence or absence of the object. A variety of fields may be used, including acoustic fields and a wide range of electromagnetic fields such as light, infrared, microwave, and electrostatic or magnetic fields. The field sensing proximity switches may be distinguished from the simple mechanical microswitch in that perturbations of the field are measured in terms of an analog electrical signal. This analog electrical signal may be displayed in analog form, but it is typically compared to a user-adjustable threshold to give a binary signal indicative of whether the object is within a predetermined distance from the field transducer.
The shortcoming of relatively simple and inexpensive field sensing proximity switches is that they typically have a single threshold. An integrated circuit called a tone decoder, type 567 such as National Semiconductor part no. LM567, for example, has a field generating oscillator and a synchronous detector all on the same chip. But the threshold detector has a fixed threshold generating a single binary output. Although the threshold level may be effectively varied by an input attentuator, such as is commonly used, there is still only a single binary output available. Thus it is desirable to have a plurality of binary outputs, each output associated with a different threshold.
Another problem associated with this simple type of threshold detector is that the threshold is set internally to the integrated circuit and may drift slightly over time. Since only a single binary output is available, it is difficult to tell from this binary output whether the threshold has changed or is set to its optimum position during the use of the proximity detector in an operating machine.